Method and device for attenuating light as a function of intensity

ABSTRACT

An intensity-dependent light modulating device and method involves light successively passing through a polarizer, a first phase retardation plate and a first light modulator. The light then strikes a mirror and then passes once again through the light modulator and the phase retardation plate in the opposite direction, and then strikes an analyzer that is crossed relative to the polarizer. The light modulator and the phase retardation plate are respectively located in an electric field in which the indicatrices of the light modulator and the phase retardation plate are deflected, wherein the deflection of the indicatrix of the light modulator is intensity-dependent. Due to the passage of light through the phase retardation plate and the light modulator, an intensity-dependent rotation of the direction of polarization occurs in such a way that higher intensity light is filtered in the analyzer while lower intensity light is transmitted.

CROSS-REFERENCE TO PRIOR APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/479,325 now U.S. Pat. No. 7,050,213 filed on Apr. 28, 2004.

FIELD OF THE INVENTION

The present invention relates to a method and a device for attenuatinglight as a function of intensity. The present invention further relatesto using a polarizer, which allows light that is linearly polarized inan input polarization direction to pass through, and an analyzercrossing the same by a substantial number of degrees (or by asubstantial angle) or by 90 degrees.

BACKGROUND OF THE INVENTION

In the same way as the human eye, light sensors, such as video camerasand CCDs, are only able to effectively process light up to a certainintensity. Very intense light sources, such as the sun, lightning bolts,or powerful artificial lamps dazzle the eye or produce glare for thelight sensor. At too high of a light intensity, overloading of the eyeor of the light sensor can result in irreparable damage or destruction.

For the human eye, this not only means that within the image of thepowerful light source only a super-bright shine is still visible and notany structures, but also that the image is swamped out or blanketed bythe shine in an entire surrounding area.

These effects are not desired in welding work, where the welding flameswamps out, distorts or diffuses details of the weld. They can also beharmful to pilots and soldiers, for example, who are blinded by the sun,stun grenades or muzzle flashes and, consequently, are no longer able tofollow the events taking place. Video cameras are also dazzled orblinded and, in fact, often to a much greater extent than the human eye.A reason for this is that the dynamic range of video cameras is smallerthan that of the human eye. And, when CCDs (charge-coupled devices) areused, one also encounters the disturbing effect that pixels, illuminatedwith too high of a light intensity, tend to “overflow”. In other words,i.e., adjacent pixels are able to register an intensity far greater thanthat actually incident on them, thereby resulting in considerableimperfections in the image.

The eye has a natural intensity-dependent attenuation device, namely theiris, which protects the eye at a high light intensity by narrowing thepupil. Cameras and other light sensors can also be equipped with anintensity-dependent attenuation device, e.g., with an adjustablediaphragm. Another attenuation possibility provides for connectingfilters in series, e.g., a pair of polarization filters, which arerotated with respect to one another by a specific intensity-dependentangle.

A disadvantage of attenuation devices of this kind arises from theirinertia. In such, an effective glare protection, for example, againstrapidly lighting-up or flashing bright light sources, such as lightningbolts, is not provided.

A further drawback of attenuation devices of this kind is that they onlyattenuate the light integrally, i.e., the entire field-of-view isuniformly darkened by a specific factor, so that bright zones and darkerzones of the field-of-view are darkened by the same factor. This canmean that, to prevent a bright zone of the field-of-view, such as thesun, from blinding the viewer or the light sensor, the field-of-viewmust be darkened or dimmed to the point where nothing more isrecognizable in darker zones of the field-of-view. In photography, thiseffect is termed the “against-the-light effect”.

For that reason, special attenuation devices have been developed whichdo not reduce the integral light intensity, but rather darken thefield-of-view only locally, where high-intensity light actually falls,while the remaining zones are not darkened or are only slightlydarkened. If the field-of-view includes the sun, for example, and thesky surrounding it, then, using such a special attenuation device, onlythe sun itself is substantially darkened, not, however, the sky, so thatthere is no more “against-the-light effect”. Such special attenuationdevices are implemented with the aid of optically addressable, spatiallyresolving light modulators (OASLM).

A light modulator (OASLM) of this kind includes a birefringent layer,whose indicatrix rotates out of a rest-position direction by a specificangle in response to the application of an external electric field tothe layer. It is assumed here that the magnitude of the electric fieldstrength is E. The electric field is generated by applying a voltage toa plate capacitor, between whose plates the light modulator (OASLM) islocated.

The polarity of the electric field is given by the polarity of thevoltage. For that reason, the magnitude of the electric field strengthcan be +E or −E. The direction of rotation of the indicatrix out of itsrest-position direction is dependent on the polarity of the electricfield (+E or −E) and reverses when the polarity of the field-generatingvoltage is reversed. For one polarity reversal, however, the magnitudeof the rotation remains unchanged.

The magnitude of the rotation depends not only on the electric fieldstrength, but, in particular, also on the intensity of the light passingthrough the birefringent layer: the magnitude of the angle of rotationincreases with the light intensity, however, at a given field strength,a specific maximum angle of rotation, referred to in the following asmaximum angle, not being exceeded in response to further increasinglight intensity. The maximum angle is dependent on the field strength.Therefore, the direction of the indicatrix is only the same over theentire surface of the light modulator when the light intensity is alsouniformly distributed over this surface. Otherwise, zones having adifferently directed indicatrix form in the light modulator; in zones ofvery great light intensity, it rotates by approximately the maximumangle, while in zones of low light intensity, only by small amounts ascompared to the rest-position direction.

German Application No. DE-OS 196 16 323 A1 refers to utilizing thiseffect to manufacture an attenuation device that darkens thefield-of-view locally, only where high-intensity light actually falls.Here, one takes advantage of the fact that the polarization direction oflinearly polarized light, which passes through a λ/2 plate, is invertedwith respect to the indicatrix of the birefringent material. For thatreason, the thickness of the birefringent layer of the light modulatoris selected in such a way as to enable the light modulator to act as aλ/2 plate. The strength of electric field E is selected so as to enablea maximum angle of 45° to be attained.

A polarizer is positioned upstream from the light modulator in such away that the polarization direction of the light transmitted by passingthrough the polarizer forms an angle of 45° with the rest-positiondirection. In addition, an analyzer is placed downstream from the lightmodulator. It is situated so as to be crossed with respect to thepolarizer by 90°.

An optical system is used to image a field-of-view onto the lightmodulator that contains, for example, a very bright light source againsta dark background. Therefore, a bright spot, namely the image of thebright light source, and a dark zone, namely the image of thebackground, are formed on the light modulator.

The electric field E is only able to rotate the indicatrix of the lightmodulator in the area of the bright spot by the maximum angle, i.e., by45°, out of the rest-position direction. Therefore, in the area of thebright spot, depending on the polarity of the electric field (+E or −E),the indicatrix is either at an angle of 0° or at an angle of 90° to thedirection of polarization of the light that is incident on thepolarizer.

As mentioned above, when passing through a λ/2 plate, the polarizationdirection is inverted with respect to the direction of the indicatrix.Since, in the area of the bright spot, the angle between thepolarization direction of the incident light and the indicatrix is 0° or90°, the polarization direction, in response to the inversion, eitherpasses into itself or is rotated by 180°, so that the only light leavingthe bright spot is light whose polarization is rotated with respect tothe input polarization either not at all (0°) or by 180°. In both cases,i.e., for every polarity of the electric field (+E or −E), the analyzer,which is crossed relatively thereto, performs a filtering-out function,so that the image of the bright light source is completely suppressed.

Another situation arises for the dark background. The electric field (+Eor −E) is only able to rotate the indicatrix of the light modulator inthe area of the dark zone by a small angle out of the rest-positiondirection. In the area of the dark zone, the indicatrix forms an anglewith the input polarization that, in each instance, varies only slightlyfrom 45° for both polarities of the electric field (+E or −E).Therefore, as a result of the inversion with respect to the direction ofthe indicatrix, the input polarization is rotated by approximately twicethis angle, thus by approximately 90°. The analyzer allows thispolarization direction to pass through, so that the image of the darkbackground can be viewed with almost undiminished intensity.

Since the birefringent material of the light modulator is a liquidcrystal, e.g., a nematic or smectic liquid crystal, an electrolysis,thus an electrochemical decomposition of the material, begins inresponse to the application of an electric field. To prevent this,instead of a constant field (+E or −E), an alternating field is appliedby continually reversing the polarity of the field strength using aspecific operating frequency of between +E and −E.

To the extent possible, the field strength preferably has a square-wavecharacteristic, so that it alternates between constant values +E and −E,the transition times preferably being kept as short as possible. Asmentioned above, both for the two polarities of the electric field,i.e., both for +E as well as for −E, the above system also has theeffect of suppressing the bright light source, not, however, the darkbackground. In this connection, the indicatrix in the area of the brightspot rotates with the operating frequency, with respect to therest-position direction, back and forth between the positive and thenegative maximum angle, which is equivalent in terms of absolute value(here ±45°).

The rotation of the molecules or molecular parts of the liquid crystal,whose orientation is decisive for the direction of the indicatrix, isencumbered, however, with relatively substantial inertia for all liquidcrystals that can be used to attain large maximum angles of 45°. Thismeans that the indicatrix exhibits a relatively long response time tochange of light. Typical values may be approximately 1/100 seconds.Light sources that light up suddenly, such as lightning bolts, can,therefore, not be suppressed quickly enough to prevent dazzling of theeye or glare for the sensor.

In response to every polarity reversal of the electric field, thelight-attenuating effect is temporarily lost, since the polarityreversal causes the indicatrix to also pass through the rest-positiondirection in the area of the bright spot. For that reason, a longresponse time entails the further disadvantage that, following eachpolarity reversal of the electric field, a relatively long time passesuntil the light-attenuating effect is achieved again.

The German Patent Application No. DE-OS 196 16 323 A1 refers to anattenuation device that can do without a maximum angle of 45° becausetwo light modulators are arranged in series. Nevertheless, in thisconfiguration as well, relatively large maximum angles are necessary, sothat there is also the drawback here that only liquid crystals having arelatively slow response time are able to be used.

However, there are a number of types of liquid crystals whose indicatrixhas a very short response time to change of light. Using these liquidcrystals, it is not possible, however, to achieve maximum angles largeenough to be used in the systems proposed by the German Application No.DE-OS 196 16 323 A1.

A further drawback of the mentioned attenuation devices is that only onepolarization direction of the incident light is utilized, while theother polarization direction is filtered out at the input polarizer.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to devising anattenuation device which makes do with just one light modulator, whichpermits the use of liquid crystals whose indicatrices have a very shortresponse time to change of light, which enables bright light sources tobe completely suppressed, even when working with a small maximum angle,and which is capable of utilizing both polarization directions of theincident light.

Embodiments of the present invention are directed to anintensity-dependent attenuation device for light, having a polarizer,which allows linearly polarized light to propagate through in one inputpolarization direction, and having an analyzer which is crossedrelatively thereto, characterized by a first and a second λ/2phase-retardation plate and a first optically addressable, spatiallyresolving light modulator designed as a λ/2-phase retardation plate,incident light propagating, in turn, through the polarizer, the firstλ/2 phase-retardation plate, the first light modulator, and the secondλ/2 phase-retardation plate, and, from there, impinging on the analyzer,

-   a) the first λ/2 phase-retardation plate having a first indicatrix    which    -   without an external electric field, runs in a first        rest-position direction;    -   in response to the application of a first external electric        field, runs in a first deflection direction, which is rotated        with respect to the first rest-position direction by a first        angle that is independent of the light intensity, about the        direction of the first external electric field; and    -   in response to the application of a second external electric        field, equal and opposite to the first external electric field,        runs in a second deflection direction, which is rotated with        respect to the first rest-position direction by a second angle,        equal and opposite to the first angle;-   b) the first light modulator having a second indicatrix, which    -   without an external electric field, runs in a second        rest-position direction;    -   in response to the application of a third external electric        field, runs in a third deflection direction, which is rotated        with respect to the second rest-position direction by a third        angle, about the direction of the third external electric field;    -   in response to the application of a fourth external electric        field, equal and opposite to the third external electric field,        runs in a fourth deflection direction, which is rotated with        respect to the second rest-position direction by a fourth angle,        equal and opposite to the third angle;        -   the magnitude of the third and of the fourth angle            increasing in response to increasing light intensity, up to            a specific maximum value, not however beyond this, and the            third and fourth deflection directions reaching a first and            second limiting deflection direction, respectively, in the            case that the magnitude of the third or fourth angle reaches            the maximum value;-   c) the second λ/2 phase-retardation plate having a third indicatrix    which    -   without an external electric field, runs in the first        rest-position direction;    -   in response to the application of a fifth external electric        field, runs in the first deflection direction; and    -   in response to the application of an external sixth electric        field, equal and opposite to the fifth external electric field,        runs in the second deflection direction;-   d) the first and the second phase-retardation plate and the first    light modulator being so oriented to one another and with respect to    the polarizer that    -   the first deflection direction and the first limiting deflection        direction run in parallel to one another and in parallel or        perpendicularly to the input polarization direction, or        -   the second deflection direction and the second limiting            deflection direction run in parallel to one another and in            parallel or perpendicularly to the input polarization            direction,-   e) the first or second external electric field is applied to the    first phase-retardation plate; the third or fourth external electric    field is applied to the first light modulator; and the fifth or    sixth external electric field is applied to the second    phase-retardation plate; and-   f) the first and second, the third and fourth, and the fifth and    sixth external electric fields, respectively, are selected in such a    way that first angle α and maximum value βmax fulfill the condition    2α−βmax=n·45°+T, n being equal to 0,1,2,3, . . . and T being a    tolerance of ±5°.

Embodiments of the present invention are directed to a method forattenuating light as a function of intensity, the light passing througha polarizer (P1), which allows linearly polarized light to propagatethrough in one input polarization direction (EPR1, EPR2) and strike ananalyzer (P2) that is crossed thereto by a substantial number of degrees(or by a substantial angle) or by 90 degrees,

the light propagating, in turn, through polarizer (P1), a first λ/2phase-retardation plate (1), a first optically addressable, spatiallyresolving light modulator (3) designed as a λ/2 phase-retardation plate,and a second λ/2 phase-retardation plate (3), and, from there, impingingon analyzer (P2),

-   a) the first λ/2 phase-retardation plate (1) having a first    indicatrix which    -   without an external electric field, runs in a first        rest-position direction (R1);    -   in response to the application of a first external electric        field (+E1), runs in a first deflection direction (A), which is        rotated with respect to the first rest-position direction (R1)        by a first angle (+α) that is independent of the light        intensity, about the direction of first external electric field        (+E1); and        -   in response to the application of a second external electric            field (−E1), equal and opposite to first external electric            field (+E1), runs in a second deflection direction (B),            which is rotated with respect to first rest-position            direction (R1) by a second angle (−α), equal and opposite to            first angle (+α);-   b) first light modulator (2) having a second indicatrix, which    -   without an external electric field, runs in a second        rest-position direction (R2);    -   in response to the application of a third external electric        field (+E2), runs in a third deflection direction (C), which is        rotated with respect to second rest-position direction (R2) by a        third angle (+β), about the direction of the third external        electric field (+E2);    -   in response to the application of a fourth external electric        field (−E2), equal and opposite to third external electric field        (+E2), runs in a fourth deflection direction (D), which is        rotated with respect to second rest-position direction (R2) by a        fourth angle (−β), equal and opposite to third angle (+β);    -   the magnitude of the third and of the fourth angle (+β, −β)        increasing in response to increasing light intensity, up to a        specific maximum value (βmax), not however beyond this, and        third and fourth deflection directions (C, D) reaching a first        and second limiting deflection direction (Cmax, Dmax),        respectively, in the case that the magnitude of third or fourth        angle (+β, −β) reaches maximum value (βmax);-   c) the second λ/2 phase-retardation plate (3) having a third    indicatrix which    -   without an external electric field, runs in the first        rest-position direction (R1);    -   in response to the application of a fifth external electric        field (+E3), runs in first deflection direction (A); and        -   in response to the application of an external sixth electric            field (−E3), equal and opposite to fifth external electric            field (+E3), runs in second deflection direction (B);-   d) first and the second phase-retardation plates (1, 3) and first    light modulator (2) being so oriented to one another and with    respect to polarizer (P1) that    -   first deflection direction (A) and first limiting deflection        direction (Cmax) running in parallel to one another and in        parallel or perpendicularly to input polarization direction        (EPR1, EPR2), or    -   second deflection direction (B) and second limiting deflection        direction (Dmax) running in parallel to one another and in        parallel or perpendicularly to input polarization direction        (EPR1, EPR2);-   e) first or second external electric field (+E1, −E1) being applied    to first phase-retardation plate (1), third or fourth external    electric field (+E2 −E2) being applied to first light modulator (2),    and fifth or sixth external electric field (+E3, −E3) being applied    to second phase-retardation plate (3); and-   f) first and second, third and fourth, and fifth and sixth external    electric fields (+E1, −E1, +E2, −E2, +E3, −E3), respectively, being    selected in such a way that first angle (α) and maximum value (βmax)    fulfill the condition 2α−βmax=·45°+T, n being equal to 0,1,2,3, . .    . and T being a tolerance of ±5°.

Embodiments of the present invention may permit a direct straight-linetransmitted-light operation, i.e., the light may enter into the systemon one side and emerge again on the opposite side in parallel to thedirection of incidence.

As when using binoculars, the sightline of the eye conforms in this caseto the sightline of the system (the optical assemblage), making iteasier to manually sight objects.

In one specific embodiment, the first and the second λ/2phase-retardation plates and the first light modulator are positioned inparallel with one another.

The first and second external electric fields, respectively, arepreferably generated by a first plate capacitor, between whose platesthe first phase-retardation plate is situated. The third and fourthexternal electric fields, respectively, are preferably generated by asecond plate capacitor, between whose plates the first light modulatorplate is situated. The fifth and sixth external electric fields,respectively, are preferably generated by a third plate capacitor,between whose plates the second phase-retardation plate is situated.

Embodiments of the present invention are directed to anintensity-dependent attenuation device for light, having a polarizer,which allows linearly polarized light to propagate through in one inputpolarization direction, and having an analyzer which is crossedrelatively thereto, characterized by a first λ/2 phase-retardationplate, a second optically addressable, spatially resolving lightmodulator designed as a λ/4 phase-retardation plate, and a mirror,incident light propagating, in turn, through the polarizer, the firstλ/2 phase-retardation plate, and the second light modulator, and, fromthere, impinging on the mirror, and, subsequently, in the reversedirection, again propagating through the second light modulator and thefirst λ/2 phase-retardation plate and, from there, impinging on theanalyzer,

-   a) the first λ/2 phase-retardation plate having a first indicatrix    which    -   without an external electric field, runs in a first        rest-position direction;    -   in response to the application of a first external electric        field, runs in a first deflection direction, which is rotated        with respect to the first rest-position direction by a first        angle that is independent of the light intensity, about the        direction of the first external electric field; and    -   in response to the application of a second external electric        field, equal and opposite to the first external electric field,        runs in a second deflection direction, which is rotated with        respect to the first rest-position direction by a second angle,        equal and opposite to the first angle;-   b) the second light modulator having a fourth indicatrix, which    -   without an external electric field, runs in a second        rest-position direction;    -   in response to the application of a seventh external electric        field, runs in a third deflection direction, which is rotated        with respect to the second rest-position direction by a third        angle, about the direction of the seventh external electric        field;    -   in response to the application of an eighth external electric        field, equal and opposite to the seventh external electric        field, runs in a fourth deflection direction, which is rotated        with respect to the second rest-position direction by a fourth        angle, equal and opposite to the third angle;        -   the magnitude of the third and of the fourth angle            increasing in response to increasing light intensity, up to            a specific maximum value, not however beyond this, and the            third and fourth deflection directions reaching a first and            second limiting deflection direction, respectively, in the            case that the magnitude of the third or fourth angle reaches            the maximum value;-   c) the first and the second phase-retardation plate and the first    light modulator being so oriented to one another and with respect to    the polarizer that    -   the first and the third deflection direction run in parallel to        one another and in parallel or perpendicularly to the input        polarization direction, or    -   the second and the fourth deflection directions run in parallel        to one another and in parallel or perpendicularly to the input        polarization direction;-   d) the first or second external electric field is applied to the    first phase-retardation plate, and the seventh or eighth external    electric field is applied to the second light modulator; and-   e) the first and second, and the seventh and eighth external    electric fields, respectively, are selected in such a way that first    angle α and maximum value βmax fulfill the condition    2α−βmax=n·45°+T, n being equal to 0,1,2,3, . . . and T being a    tolerance of ±5°.

Embodiments of the present invention are directed to a method forattenuating light as a function of intensity, the light passing througha polarizer (P1), which allows linearly polarized light to propagatethrough in one input polarization direction (EPR1, EPR2) and strike ananalyzer (P2, ST1, ST2, ST3) that is crossed thereto by a substantialnumber of degrees (by a substantial angle) or by 90 degrees,characterized by a first λ/2 phase-retardation plate (1), a secondoptically addressable, spatially resolving light modulator (4) designedas a λ/4 phase-retardation plate, and a mirror (5), incident light (10a, 10 b) propagating, in turn, through polarizer (P1, ST1, ST2, ST3),first λ/2 phase-retardation plate (1), and second light modulator (4),and, from there, impinging on mirror (5), and, subsequently, in thereverse direction, again propagating through second light modulator (4)and first λ/2 phase-retardation plate (1) and, from there, impinging onanalyzer (P2, ST1, ST2, ST3),

-   a) the first λ/2 phase-retardation plate (1) having a first    indicatrix which    -   without an external electric field, runs in a first        rest-position direction (R1);    -   in response to the application of a first external electric        field (+E1), runs in a first deflection direction (A), which is        rotated with respect to the first rest-position direction (R1)        by a first angle (+α) that is independent of the light        intensity, around the direction of first external electric field        (+E1); and    -   in response to the application of a second external electric        field (−E1), equal and opposite to first external electric field        (+E1), runs in a second deflection direction (B), which is        rotated with respect to first rest-position direction (R1) by a        second angle (−α), equal and opposite to first angle (+α);-   b) second light modulator (4) having a fourth indicatrix which    -   without an external electric field, runs in a second        rest-position direction (R2);    -   in response to the application of a seventh external electric        field (+E4), runs in a third deflection direction (C), which is        rotated with respect to second rest-position direction (R2) by a        third angle (+β), around the direction of seventh external        electric field (+E4);    -   in response to the application of an eighth external electric        field (−E4), equal and opposite to seventh external electric        field (+E4), runs in a fourth deflection direction (D), which is        rotated with respect to second rest-position direction (R2) by a        fourth angle (−β), equal and opposite to third angle (+β);    -   the magnitude of the third and of the fourth angle (+β, −β)        increasing in response to increasing light intensity, up to a        specific maximum value (βmax), not however beyond this, and        third and fourth deflection directions (C, D) reaching a first        and second limiting deflection direction (Cmax, Dmax),        respectively, in the case that the magnitude of third or fourth        angle (+β, −β) reaches maximum value (βmax);-   c) first and second phase-retardation plates (1, 2) and first light    modulator (3) being so oriented to one another and with respect to    polarizer (P11, ST1, ST2, ST3) that    -   first deflection direction (A) and first limiting deflection        direction (Cmax) run in parallel to one another and in parallel        or perpendicularly to input polarization direction (EPR1, EPR2),        or        -   second deflection direction (B) and second limiting            deflection direction (Dmax) running in parallel to one            another and in parallel or perpendicularly to input            polarization direction (EPR1, EPR2);-   d) first or second external electric field (+E1, −E1) being applied    to first phase-retardation plate (1), and seventh or eighth external    electric field (+E4, −E4) being applied to second light modulator    (4); and-   e) first and second, seventh and eighth external electric fields    (+E1, −E1, +E4, −E4), respectively, being selected in such a way    that first angle (α) and maximum value βmax) fulfill the condition    2α−βmax=n·45°+T, n being equal to 0,1,2,3, . . . and T being a    tolerance of ±5°.

Due to the reflection of the light at the mirror, the second opticallyaddressable, spatially resolving light modulator, designed as a λ/4phase-retardation plate, is passed through twice and, due to the phasejump associated with the reflection, acts on the polarization directionof the light as an optically addressable, spatially resolving lightmodulator that is designed as a λ/2 phase-retardation plate. A secondλ/2 phase-retardation plate is not necessary in this system (opticalassemblage), since the first λ/2 phase-retardation plate is likewisepassed through twice.

This system and, respectively, this method do not permit anystraight-line transmitted-light operation. However, an indirect,straight-line, transmitted-light operation may be enabled by usingsuitable light-deflection devices, e.g., additional mirrors.

In a further embodiment, this specific embodiment includes a polarizingbeam splitter, which combines the polarizer and analyzer, thusincorporating both. In this connection, the polarizing beam splitteracts as a polarizer for incident light and, at the same time, as ananalyzer for light reflected at the mirror.

In addition, the use of a polarizing beam splitter makes it possible tonot only subject one polarization direction to an intensity-dependentattenuation in accordance with the present invention and to block thepolarization direction that is perpendicular thereto using thepolarizer, but also to subject both polarization directions at the sametime to an intensity-dependent attenuation in accordance with thepresent invention, so that no polarization direction needs to beblocked. Thus, this specific embodiment of the present invention worksindependently of the polarization of the light to be attenuated.

In this specific embodiment of the present invention, the incident lightpropagating through the polarizing beam splitter is divided into alinearly polarized first beam component, which is deflected in thepolarizing beam splitter, and into a second beam component, which islinearly polarized perpendicularly to the first beam component and isnot deflected in the polarizing beam splitter. At this point, both beamcomponents are subjected, separately from one another, to anintensity-dependent attenuation in accordance with the presentinvention. In the process, both the first, as well as the second beamcomponents pass, separately from one another, through first λ/2phase-retardation plate and the second light modulator, and impinge onthe mirror. From there, in reverse order, they pass again through thesecond light modulator and the first λ/2 phase-retardation plate, andarrive again in the polarizing beam splitter.

There, the first beam component is divided into a third beam componentwhich is linearly polarized in parallel to the first beam component andis deflected in the polarizing beam splitter, and into a fourth beamcomponent which is linearly polarized perpendicularly to the first beamcomponent and is not deflected in the polarizing beam splitter.

After once again entering into the polarizing beam splitter, the secondbeam component is divided into a fifth beam component, which is linearlypolarized at right angles to the second beam component and is deflectedin the polarizing beam splitter, and into a sixth beam component whichis linearly polarized in parallel to the second beam component and isnot deflected in the polarizing beam splitter. Thus, the sixth beamcomponent is directed in an opposite, parallel direction to the incidentlight. The fourth and the fifth beam components are polarizedperpendicularly to one another.

The fourth and/or the fifth beam components are supplied to a monitoringor analysis, since they are deflected oppositely to the incident light.The sum of the intensities of the fourth and fifth beam components isdependent, in accordance with the present invention, not on thepolarization state, but rather only on the intensity of the incidentlight. Thus, the information contained in the polarization state of theincident light is not lost, but rather is contained in the sum of thefourth and fifth beam components.

In one specific embodiment, the first λ/2 phase-retardation plate, thesecond light modulator, and the mirror are positioned in parallel to oneanother, so that normally (or perpendicularly) incident light isreflected into itself.

One or more light-deflecting surfaces may be configured between thepolarizing beam splitter and the first λ/2 phase-retardation plate todeflect the first and the second beam components, respectively, in a waythat enables them to run in parallel to one another.

In addition, the polarizing beam splitter itself may have at least onelight-deflecting surface which deflects the first and the second beamcomponent, respectively, in a way that enables them to run in parallelto one another.

In a refinement of this specific embodiment, the polarizing beamsplitter is formed and positioned in such a way that the path of thefourth beam component coincides with the path of the fifth beamcomponent, so that, together, they are able to be supplied to amonitoring or evaluation.

To suppress stray light, a collimator may be placed upstream from themirror.

One specific embodiment of the present invention provides for a lens,which images a field-of-view onto the light modulator. In a furtherrefinement of this specific embodiment, an eyepiece is positioned in away that enables the image of the field-of-view to be observed using theeyepiece. Thus, such a specific embodiment of the present invention is atype of telescope which attenuates the intensity of bright points of thefield-of-view percentually more than that of less bright points.

In accordance with the present invention, those λ/2 phase-retardationplates are used which have an indicatrix whose direction may beinfluenced by applying an electric field, this direction not dependingon the light intensity and being the same over the entire surface of theλ/2 phase-retardation plate. The rotation of the indicatrix is based ona reorientation of molecules or of molecular parts, stimulated by theelectric field, the macroscopic orientation of the λ/2 phase-retardationplate itself remaining unaffected. λ/2 phase-retardation plates of thiskind have been available.

Embodiments of the present invention provide for a light modulator to beused which, in contrast to the λ/2 phase-retardation plates, has anindicatrix whose direction may be influenced both by applying anelectric field, as well as by the light intensity. Such light modulatorsare referred to as “optically addressable, spatially resolving lightmodulators” or “OASLMs.” Embodiments of the present invention providefor using such a light modulator as a non-linear optical filter.

In this connection, a light modulator having a birefringentliquid-crystal layer is preferably used, which, for example, may be aliquid-crystal film of chiral smectic-C material, whose molecules have aferroelectric, deformable helix.

In a constant electric field, electrolysis causes liquid crystals todecompose. To prevent this, one specific embodiment provides forapplying an AC (alternating current) voltage having a substantiallysquare-wave time characteristic to the third or fourth plate capacitor,so that the third and the fourth, and the seventh and the eighthelectric field, respectively, continually replace one another.

Thus, the polarity of the electric field in the third or fourthcapacitor, i.e., the electric field applied to the first or second lightmodulator, is continually reversed. In response to each polarityreversal, the second and fourth indicatrices, respectively, rotate backand forth between third angle +β and fourth angle −β, these angles beingin relation to the particular rest-position direction of the second andfourth indicatrices, respectively, and, at a high light intensity, themagnitude of angles +β and −β being able to attain, at the most, maximumvalue βmax.

In this case, the angular condition 2α−βmax=n·45°+T is only fulfilledfor one polarity of the electric field in the second and fourth platecapacitor, respectively. For that reason, in the case that the polarityof the electric field applied to the first or second light modulator isreversed, the polarity of the electric field applied to the λ/2phase-retardation plate(s) is also reversed in phase therewith, so thatthe first and third indicatrices and the first indicatrix, respectively,rotate back and forth between first angle +α and second angle −α, eachtime in relation to the rest-position direction of the first and thirdindicatrices, respectively, thereby fulfilling the angular condition forboth polarities. This takes place in that an AC voltage having asubstantially square-wave characteristic is also applied to the firstand third plate capacitor, respectively.

Therefore, in this specific embodiment of the present invention, thezero crossings of all AC voltages occur simultaneously. In one specificembodiment, all AC voltages originate from a common voltage source,where the individual field strengths may be adjustable by using suitablevoltage-adjusting devices, such as potentiometers.

The first and second external electric fields, respectively, arepreferably generated by a first plate capacitor, between whose platesthe first phase-retardation plate is situated. The seventh and eighthexternal electric fields, respectively, are preferably generated by afourth plate capacitor, between whose plates the second light modulatoris situated.

The plates of the plate capacitors may be constituted of electricallyconductive layers which are at least partially reflecting (or partiallytransparent) to light, such as of indium tin oxide, and be directlyplaced on the light-transit surfaces of the phase-retardation plates orof the light modulators, or be placed at a distance, in parallel to thesame, using a transparent substrate material. In another specificembodiment, the layers are made of a thin layer of metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-sectional representation of a specificembodiment of a device according to the present invention fortransmitted-light operation, inclusive of the electrical wiring, as wellas of the electric fields generated by the same, a high-intensity beamof light being incident.

FIG. 2A shows a schematic perspective representation of the device ofFIG. 1, at the same point in time as in FIG. 1, the electrical wiringbeing omitted.

FIG. 2B shows the device of FIG. 2A, the polarity of the electric fieldsbeing the reverse of that in FIG. 2A.

FIG. 3 shows the device of FIGS. 2A and 2B, respectively, alow-intensity beam of light being incident.

FIG. 4 shows a cross-sectional representation of another specificembodiment of the present invention, inclusive of the electrical wiringand of the electric fields generated by the same.

FIG. 5A shows a schematic perspective representation of the device ofFIG. 4, onto which a high-intensity beam of light falls, the electricalwiring being omitted.

FIG. 5B shows the specific embodiment of FIG. 5A, a low-intensity beamof light being incident.

FIG. 6 shows a schematic representation of an embodiment of a deviceaccording to the present invention, having a polarizing beam splitter, alow-intensity beam of light being incident thereon, the electricalwiring being omitted.

FIG. 7 shows a schematic representation of another specific embodimentof a device according to the present invention, the electrical wiringbeing omitted.

FIG. 8 shows a schematic representation of another specific embodimentof a device according to the present invention, the electrical wiringbeing omitted.

FIG. 9A shows another specific embodiment of a device according to thepresent invention, the electrical wiring being omitted, viewed in adirection at right angles to the incoming and at right angles to theemergent light beam.

FIG. 9B shows the device of 9A, viewed in the direction of the incidentlight beam.

FIG. 9C shows the device of FIG. 9A, viewed in the opposite direction ofthe emergent light beam.

DETAILED DESCRIPTION

FIG. 1 shows a cross-sectional representation of a specific embodiment adevice according to the present invention for transmitted-lightoperation, inclusive of the electrical wiring and of the electric fieldsgenerated by the same. A light beam 10 a of high intensity is incidenton polarizer P1. The light component transmitted or admitted by thispolarizer passes through a first λ/2 phase-retardation plate 1, a firstoptically addressable, spatially resolving light modulator 2 designed asa λ/2 phase-retardation plate, and a second λ/2 phase-retardation plate3, and impinges on an analyzer P2 that is crossed with respect topolarizer P1.

First λ/2 phase retardation plate 1, light modulator 2, and second λ/2phase-retardation plate 3 are arranged between the plates of a platecapacitor C1, C2, C3, respectively. The plates of plate capacitors C1,C2, C3 are constituted of light-transmitting (or transparent),electrically conductive layers which are made of a thin layer ofvapor-deposited metal or of indium tin oxide.

Using a voltage source 20 and potentiometers W1, W2, W3, a voltage U1 isapplied to first plate capacitor C1, a voltage U2 to second platecapacitor C2, and a voltage U3 to third plate capacitor C3, so that anelectric field is produced in each of plate capacitors C1, C2, C3.

First λ/2 phase-retardation plate 1, light modulator 2, and second λ/2phase-retardation plate 3 preferably contain liquid crystals, whichdecompose in an electric field of constant polarity, as the result ofelectrolysis. To prevent this, voltage source 20 supplies an AC voltageUo preferably having a substantially square-wave time characteristic, sothat the polarity of voltages U1, U2, U3 and thus also the electricfields are continually reversed in plate capacitors C1, C2, C3. Thesquare-wave time characteristic of the AC voltage is preferably selectedin such a way that the transition times are kept as short as possiblewhen switching over between positive and negative polarity or viceversa. The advantage of short transition times is elucidated below.

First potentiometer W1 is adjusted in such a way that first λ/2phase-retardation plate 1 in plate capacitor C1 is alternately situatedin a first external electric field +E1 and a second external electricfield −E1 equal and opposite hereto. Analogously to this, second andthird potentiometers W2, W3 are adjusted in such a way that lightmodulator 2 in plate capacitor C2 and second λ/2 phase-retardation plate3 in plate capacitor C3, respectively, are alternately situated in athird and in a fourth external electric field +E2, −E2 equal andopposite hereto, and, respectively, in a fifth and a sixth externalelectric field +E3, −E3 equal and opposite hereto. Electric fields +E1,+E2, +E3 are cyclically reversed in polarity by AC voltages U1, U2, U3into electric fields −E1, −E2, −E3 which are equal and opposite thereto,respectively. These polarity reversals take place simultaneously, sinceall three voltages U1, U2, U3 originate from shared voltage source 20and run analogously to AC voltage Uo.

The situation is illustrated in FIG. 1 at an instant when first λ/2phase-retardation plate 1 is situated in field −E1, light modulator 2 infield −E2, and second λ/2 phase-retardation plate 3 in field −E3.Electric fields −E1, −E2, −E3 are shown in FIG. 1 by a plurality ofarrows running to the right.

FIGS. 2A and 2B show schematic, perspective representations of thedevice of FIG. 1, the plate capacitors and their electric wiring beingomitted for the sake of clarity. Clockwise rotations are characterizedin the following by angles having a positive sign, and counter-clockwiserotations by angles having a negative sign.

Light beam 10 a of high intensity is incident on polarizer P1. This isoriented in such a way that it only allows one light component having afirst input polarization direction EPR1 to pass; reference is madefurther below to second polarization direction EPR2 that isperpendicular thereto. After the light component having polarizationdirection EPR1 has left polarizer P1, it impinges on first λ/2phase-retardation plate 1.

First λ/2 phase-retardation plate 1 has a first indicatrix which runswithout an external electric field in a first rest-position directionR1. When, however, first or second external electric field +E1 and −E1,respectively, (FIG. 1) is applied to first λ/2 phase retardation plate1, its molecules or parts thereof reorient themselves in such a way thatthe first indicatrix rotates out of first rest-position direction R1into a first or second deflection direction A and B, respectively, whichis rotated with respect to first rest-position direction R1 by a firstand, respectively, second angle +α, −α that is equal and oppositethereto, about the direction of electric field +E1 and −E1,respectively. Angles +α, −α are independent of the light intensity.

Once the light has propagated through first λ/2 phase-retardation plate1, it impinges on first light modulator 2. This has a second indicatrixwhich runs without an external electric field in a second rest-positiondirection R2. When, however, third or fourth external electric field+E2, −E2 (FIG. 1) is applied to first light modulator 2, the secondindicatrix rotates out of second rest-position direction R2 into a thirdor fourth deflection direction C and D, respectively, which is rotatedwith respect to second rest-position direction R2 by a third and,respectively, fourth angle +β, −β that is equal and opposite thereto,about the direction of electric field +E2 and −E2, respectively. Themagnitude of third and fourth angle +β and −β, respectively, increasesin response to increasing light intensity, up to a specific maximumvalue βmax, not, however, beyond this, so that third and fourthdeflection directions C and D reach first and second limiting deflectiondirections Cmax and Dmax, respectively, in the case that the magnitudeof third or fourth angle +β and −β, respectively, reaches maximum valueβmax.

In the following, to facilitate understanding, it is assumed that lightbeam 10 a is so intense that the magnitude of third and fourth angle +β,respectively, assumes maximum value βmax, so that third and fourthdeflection directions C and D conform with first and second limitingdirection Cmax and Dmax, respectively. The second indicatrix runs inthis case in the first or second limiting deflection direction Cmax andDmax, respectively.

The light subsequently impinges on second λ/2 phase-retardation plate 2.This plate has a third indicatrix, which runs without external electricfield in first rest-position direction R1 and in fifth and sixthexternal electric field +E3 and −E3, respectively, (FIG. 1) is rotatedout of first rest position direction R1 by first and second angle +α and−α, respectively, that are independent of the light intensity, intofirst and second deflection direction A and B, respectively. Thus, thethird indicatrix always runs in parallel to the first indicatrix.Therefore, in one specific embodiment of the present invention, firstand second λ/2 phase-retardation plates 1, 2 are identical in design,and electric fields −E1 and −E3 or +E1 and +E3, respectively, appliedthereto, are each of the same absolute value. After passing throughsecond λ/2 phase-retardation plate 2, the light impinges on analyzer P2,which is situated, in accordance with the present invention, so as to becrossed with respect to polarizer 1. This position of analyzer P2 ischaracterized in FIGS. 2A, 2B by a solid-line, horizontal double arrow.

The mutual orientation of input polarization direction EPR1, of firstrest-position direction R1, and of second rest-position direction R2 isexpressed in accordance with the present invention by the followingconditions,

-   (a) first angle α and maximum value βmax fulfill the condition    2α−βmax=n·45°+T, n being equal to 0,1,2,3, . . . and T being a    tolerance of ±5°.-   (b) either first deflection direction A and first limiting    deflection direction Cmax run both in parallel or perpendicularly to    input polarization direction EPR1,    -   or second deflection direction B and second limiting deflection        direction Dmax either both run in parallel or perpendicularly to        input polarization direction EPR1.

The condition mentioned under (a) is able to be fulfilled by properlyselecting external electric fields +E1 and −E1, +E2 and −E2, or +E3 and−E3, respectively. The condition named under (b) is fulfilled in thatfirst and second λ/2 phase-retardation plates 1, 3, as well as lightmodulator 2 are installed in an appropriate orientation in the device ofFIG. 2A.

FIG. 2A shows the device of FIG. 1 at the same point in time as in FIG.1, i.e., electric fields −E1, −E2, −E3 are applied.

With regard to condition (a), the specific embodiment of the presentinvention illustrated in FIG. 2A corresponds to the case n=0, T=0, theelectric fields being selected in accordance with the present inventionby setting potentiometers W1, W2, W3 (FIG. 1) in such a way that thecondition named under (a) is fulfilled. With regard to condition (b),the specific embodiment of the present invention illustrated in FIG. 2Acorresponds to the case where second deflection direction B and secondlimiting deflection direction Dmax both run in parallel to inputpolarization direction EPR1. From this is derived for n=0, T=0, thatsecond rest-position direction R2, as compared to EPR1 direction, formstwice as great an angle as first rest-position direction R1.

Generally, the polarization direction of linearly polarized light isinverted with respect to the direction of the indicatrix of the λ/2plate when passing through the same. This effect is utilized by thepresent invention, as explained in the following.

After the light component having polarization direction EPR1 has leftpolarizer P1, it impinges on first λ/2 phase-retardation plate 1. Thisplate has a first indicatrix, which is rotated out of first restposition direction R1 by a second angle −α into a second deflectiondirection B, since second electric field −E1 is being applied. Thisangle is independent of the light intensity. In addition, in accordancewith condition (a), first phase-retardation plate 1 is oriented withrespect to polarizer P1 in such a way that second deflection direction Bruns in parallel to input polarization direction EPR1.

For that reason, the light component having polarization direction EPR1and coming from polarizer P1 meets with a first indicatrix, which is inparallel to polarization direction EPR1, in first λ/2 phase-retardationplate 1. Therefore, an inversion of the polarization direction withrespect to the first indicatrix results in polarization direction EPR1passing into itself. Therefore, when passing through first λ/2phase-retardation plate 1, there is no change in the polarizationdirection.

The light subsequently impinges on first light modulator 2. Thismodulator has a second indicatrix, which is rotated out of second restposition direction R2 by a fourth angle −β, since field −E2 is beingapplied. Due to the high intensity of light beam 10 a, at the point ofincidence of the light beam on light modulator 2, fourth angle −βreaches maximum magnitude βmax, so that fourth angle −β in FIG. 2A hasvalue −βmax. Thus, the second indicatrix runs in second limitingdeflection direction Dmax.

In accordance with condition (a), first light modulator 2 is orientedwith respect to polarizer P1 in such a way that second deflectiondirection Dmax runs in parallel both to second deflection direction B,as well as to input polarization direction EPR1. Therefore, at its pointof incidence on first light modulator 2, the light, which is polarizedin the EPR1 direction, meets with the second indicatrix in parallel topolarization direction EPR1. Thus, when traversing first light modulator2, polarization direction EPR1 passes again into itself, so that thereis again no change in the polarization direction.

The light, which continues to be polarized in the EPR1 direction,subsequently impinges on second λ/2 phase-retardation plate 2. Thisplate has a third indicatrix, which is rotated out of firstrest-position direction R1 by second angle −α that is independent of thelight intensity, into second deflection direction B, since field −E3 isbeing applied. In accordance with condition (a), second deflectiondirection B runs in parallel to input polarization direction EPR1. Thus,when traversing second λ/2 phase-retardation plate 3, polarizationdirection EPR1 passes again into itself, so that there is again nochange in the polarization direction.

For that reason, the light impinges in input polarization direction EPR1on analyzer P2, which is situated in accordance with the presentinvention so as to be crossed with respect to polarizer P1, so that thelight from analyzer P2 is completely filtered out.

FIG. 2B shows the device from FIG. 2A, a high-intensity light beam 10 aagain being incident thereon, however the polarity of the electricfields being the reverse of that in FIG. 2A. Accordingly, the situationis illustrated in FIG. 2B at an instant when first λ/2 phase-retardationplate 1 is situated in field +E1, light modulator 2 in field +E2, andsecond λ/2 phase-retardation plate 3 in field +E3.

In addition it holds that condition (a) is realized for the case thatn=0, T=0 and, in accordance with condition (b), second deflectiondirection B and second limiting deflection direction Dmax both run inparallel to input polarization direction EPR1.

At the instant shown in FIG. 2B, first λ/2 phase-retardation plate 1 hasa first indicatrix, which is rotated out of first rest-positiondirection R1 into first deflection direction A, which is rotated by afirst angle +α with respect to first rest-position direction R1, sincefield +E1 is being applied. Thus, first deflection direction A isrotated by twice the magnitude of first angle +α, thus by 2α, ascompared to second deflection direction B and thus also with respect toinput polarization direction EPR1. This angle is independent of thelight intensity.

For that reason, the light component having polarization direction EPR1and leaving polarizer P1 meets with a first indicatrix, which forms anangle 2α with polarization direction EPR1, in first λ/2phase-retardation plate 1. Therefore, the inversion of the polarizationdirection with respect to the first indicatrix that occurs when firstλ/2 phase-retardation plate 1 is traversed, results in a rotation ofpolarization direction EPR1 by an angle 4α.

For that reason, after the light has passed through first λ/2phase-retardation plate 1, it impinges on first light modulator 2 in apolarization direction that is rotated by angle 4α with respect to inputpolarization direction EPR1. This modulator has a second indicatrixwhich, due to the high intensity of light beam 10 a, runs in electricfield +E2 at the point of incidence of light on light modulator 2 infirst limiting deflection direction Cmax, which is rotated with respectto second rest-position direction R2 by maximum value βmax. On the basisof condition (a), it follows for n=0 and T=0 that βmax=2α and, thus,2βmax=4α.

First limiting deflection direction Cmax, for its part, is likewiserotated by angle βmax with respect to second rest-position direction R2and, therefore, at the point of incidence of light on light modulator 2,by angle 2βmax with respect to input polarization direction EPR1. Inaccordance with the present invention, the polarization direction of thelight incident on first light modulator 2 is identical to the directionof the second indicatrix, in FIG. 2B as well. Thus, when traversingfirst light modulator 2, i.e., in response to inversion of thepolarization direction with respect to the direction of the secondindicatrix, this polarization direction does not change, rather passesinto itself. Therefore, the light leaves the first light modulator in apolarization direction, which continues to be rotated with respect toinput polarization direction EPR1 by angle 4α.

This light subsequently impinges on second λ/2 phase-retardation plate2, which has a third indicatrix, which is rotated in fifth externalelectric field +E3 out of first rest-position direction R1 by angle +α,which is independent of the light intensity, into first deflectiondirection A, which means that the third indicatrix in field +E3 forms anangle 2α with input polarization direction EPR1 (polarization direction0°).

This angle is smaller by 2α and, thus, half the size of angle 4α formedby the polarization direction of the light when impinging on second λ/2phase-retardation plate 2, with input polarization direction EPR1.Therefore, the inversion of the polarization direction with respect tothe third indicatrix results in a rotation of the polarization directionby an angle of −4α, i.e., a rotation is carried out back to inputpolarization direction EPR1.

For that reason, the light impinges in input polarization direction EPR1on analyzer P2, which is situated in accordance with the presentinvention so as to be crossed with respect to polarizer P1, so that thelight from analyzer P2 is completely filtered out.

Accordingly, in accordance with the present invention, when conditions(a) and (b) are met, the system (or optical assemblage) of FIGS. 2A and2B, respectively, always blocks a light beam 10 a of high enoughintensity independently of the momentary polarity of AC voltage Uo.

At this point, reference is made to FIG. 3, which shows the device ofFIGS. 2A and 2B, respectively, a low-intensity beam of light beam 10 bbeing incident thereon. For the sake of clarity, the situations areshown at the same time in FIG. 3, where electric fields +E1, +E2, +E3are applied, and where fields −E1, −E2, −E3 having reversed polarity areapplied to first phase-retardation plate 1, to light modulator 2, and tosecond phase-retardation plate 2, although these situations do not occursimultaneously, but rather alternating with the frequency of AC voltageUo (FIG. 1).

Initially, analogously to FIG. 2α, the case is considered where electricfields −E1, −E2 and −E3, respectively, are applied to firstphase-retardation plate 1, to first light modulator 2, and to secondphase-retardation plate 3, respectively. Light beam 10 b of lowintensity traverses polarizer P2, which only allows that component ofthe light beam which has input polarization direction EPR1 to passthrough. This light impinges on the first phase-retardation plate, whichhas a first indicatrix, which, in the same way as in the situation ofFIG. 2A, runs in second deflection direction B rotated by second angle−α counter to first rest position direction R1, since field −E1 is beingapplied, and second angle −α is independent of the light intensity.Therefore, when passing through first phase-retardation plate 1, thereis no change in the polarization direction.

On the other hand, the indicatrix of first light modulator 2, i.e., thesecond indicatrix, is deflected at the point of incidence of the lightonly by a small angle −β out of second rest-position direction R2 andruns in a fourth deflection direction D, which, in contrast to thesituation of FIG. 2A, is not in parallel to the EPR1 direction, butrather only differs slightly from second rest-position direction R2,since field −E2 is being applied, but the magnitude of angle β isintensity-dependent and light beam 10 b is of low intensity. Therefore,when the weak light beam propagates through first light modulator 3, thepolarization direction rotates by twice the angular difference βmax−β.For very strong light intensity, the magnitude of β approaches βmax, sothat, in this case, the angular difference βmax−β approaches zero, andthere is no rotation of the polarization direction which corresponds tothe situation of FIG. 2A. On the other hand, for very weak lightintensity, the magnitude of β approaches zero, so that, in this case,the polarization direction is rotated by angle 2βmax.

The third indicatrix of second phase-retardation plate 3 runs inparallel to first indicatrix, thus in EPR1 direction, since field −E3 isapplied. Therefore, the light beam strikes second phase-retardationplate 3 in a polarization direction that is rotated by angle 2·(βmax−β)counter to EPR1 direction.

As the result of inversion with respect to the third indicatrix, thispasses over into a polarization direction which is rotated by angle−2·(βmax−β) counter to the EPR1 direction, which corresponds to arotation about angle −4·(βmax−β).

For very weak light intensity, angle −2·(βmax−β) approaches value−2βmax. For the particularly favorable special case βmax=45°, a veryweak light beam exits the second phase-retardation plate, rotated by−90° counter to the EPR1 direction, and is, therefore, able to passthrough analyzer P2 without being attenuated.

Analogously to the situation depicted in FIG. 2B, in FIG. 3, the case isconsidered where electric fields +E1 and +E2 and +E3, respectively, areapplied to first phase-retardation plate 1, to first light modulator 2,and to second phase-retardation plate 3, respectively. Light havinginput polarization direction EPR1 impinges on first phase-retardationplate, whose indicatrix runs in first deflection direction A that isrotated by first angle +α counter to first rest-position direction R1and, thus, by angle +2α counter to the EPR1 direction. Therefore, whenfirst phase-retardation plate 1 is passed through—inversion of inputpolarization direction EPR1 with respect to first deflection directionA—the polarization direction is rotated by angle 4α, so that, afterpassing through first phase-retardation plate, the polarizationdirection is rotated by angle 4α counter to the EPR1 direction.

The indicatrix of first light modulator 2, i.e., the second indicatrix,is deflected at the point of incidence of the light, only by a smallangle +β, out of second rest-position direction R2, since field +E2 isapplied, but light beam 10 b is of low intensity. Therefore, at thepoint of incidence of the light, the second indicatrix runs in a thirddeflection direction C which differs by angle +β from secondrest-position direction R2 and, thus, due to condition (b), by angleβmax+β from the EPR1 direction. In addition, in accordance withcondition (a) for n=0 and T=0, angle βmax=2α, so that βmax+β=2α+β.

Therefore, the angular difference between the polarization direction ofthe light beam striking first light modulator 2, and third deflectiondirection C amounts to 2α+β−4α=−2α+β. For that reason, when traversinglight modulator 2, i.e., when inverted with respect to third deflectiondirection C, the polarization direction is rotated by angle −4α+2β.

When impinging on second phase-retardation plate 3, the polarizationdirection of the light beam is rotated by angle +2β counter to the EPR1direction. The third indicatrix runs in first deflection direction A,since external electric field +E3 is applied to second phase-retardationplate 3. Therefore, the angular difference from the polarizationdirection of the light beam impinging on second phase-retardation plate3 amounts to +2α−2β. Thus, a rotation of the polarization directionfollows—inversion with respect to the third indicatrix—by angle +4α−4β,so that the light beam leaves the second phase-retardation plate in apolarization direction that is rotated by angle +4α−4β counter to theEPR1 direction.

For very small light intensities, angle β approaches zero. In addition,due to condition (a), for n=0, T=0, thus for the specific embodiment ofthe present invention elucidated here, it holds that βmax=2α. For theparticularly favorable special case βmax=45°, a very weak light beamexits the second phase-retardation plate, rotated by 90° counter to theEPR1 direction, and is therefore able to pass through analyzer P2without being attenuated.

However, when such a large maximum value of βmax=45° is not able to beattained using the particular liquid crystals, the present invention mayalso be applied; in this case, merely the maximum possible transmissionis reduced, so that weak light beams may also be noticeably attenuated.However, since intense light beams are completely filtered out, theapplicability of the present invention is also retained in this case.Thus, one also derives the substantial benefit from the presentinvention of also being able to use those liquid crystals in the lightmodulator with which only a maximum value βmax of considerably less than45° is attainable. For βmax=30°, for example, a maximum transmission of50% is still reached.

In accordance with the present invention, when conditions (a) and (b)are met, the system (or optical assemblage) of FIG. 3 always allows avery weak light beam 10 b to pass through, independently of themomentary polarity of AC voltage Uo.

Even when angle β is greater than zero, the light beam may pass throughthe analyzer, the transmission decreasing, however, in response to anincreasing angle b, i.e., in response to an increase in the lightintensity, and, for example, at β=45° reaching the value zero and thuspassing over into the situation of FIGS. 2A and 2B, respectively, i.e.,complete blocking.

When the polarity of the fields is reversed, the indicatrices pass fromone deflection direction into another. In the transition time, condition(a) or (b) or both conditions is/are not met, so that thelight-attenuating effect of the present invention cannot be achieved.For that reason, it is advantageous to have short transition times or ashort response time of the indicatrix to change of light.

At this point, reference is made to FIGS. 4, 5A and 5B, whichschematically show another specific embodiment of the present invention.A high-intensity light beam 10 a (FIGS. 4, 5A) and a low-intensity lightbeam 10 b (FIG. 4B), respectively, strike polarizer P1. The lightcomponent transmitted or admitted by this polarizer passes through afirst λ/2 phase-retardation plate 1 and a second optically addressable,spatially resolving light modulator 4 designed as a λ/4phase-retardation plate, and subsequently impinges on a mirror 5, fromwhere the light beam, in reverse order, again propagates through thesecond light modulator and, after than, first phase-retardation plate 1,and then impinges on analyzer P2 which is crossed with respect topolarizer P1 and is positioned in this specific embodiment of thepresent invention next to polarizer P1. To enable the light beam to passgeometrically both through polarizer P1, as well as through analyzer P2,its direction of incidence on mirror 5 is not perpendicular to mirror 5;however, the deviation between the direction of incidence of the lightbeam on mirror 5, from the normal, is selected, to be so slight that theresultant geometric increase in the optical wavelengths in firstphase-retardation plate 1 and in second light modulator 4 is negligible.

First λ/2 phase-retardation plate 1 and light modulator 4 are positionedbetween the plates of plate capacitors C1 and C4, respectively, (FIG.4), whose plates are formed from light-transmitting, electricallyconductive layers. An electric field is generated in each of platecapacitors C1, C4 in response to the application of voltages U1, U4,which are adjustable by potentiometers W1, W4. Voltage source 20supplies an AC voltage Uo, preferably having a substantially square-wavetime characteristic, so that the polarity of voltages U1, U4, and thusalso of the electric fields, is continually reversed in plate capacitorsC1, C4.

Potentiometers W1, W4 are adjusted in such a way that light modulator 4in plate capacitor C4 and first λ/2 phase-retardation plate 1 in platecapacitor C1, respectively, are alternately situated in a seventh and inan eighth external electric field +E4, −E4 equal and opposite hereto,and, respectively, in a first and a second external electric field +E1,−E1 equal and opposite hereto. Electric fields +E1, +E4 are cyclicallyreversed in polarity by AC voltages U1, U4 and, at the same time, intoelectric fields −E1, −E4 which are equal and opposite thereto,respectively, and vice versa. The situation is illustrated in FIGS. 4and 5A at an instant when first λ/2 phase-retardation plate 1 issituated in field −E1 and second light modulator 4 in field −E4.

Field strengths −E1, +E1, −E4 and +E4 are selected in such a way, andpolarizer P1, first phase-retardation plate 1 and second light modulator4 are positioned in such a way in accordance with the present inventionthat conditions (a) and (b) are met.

Second light modulator 4 has a fourth indicatrix, which runs without anexternal electric field in a second rest-position direction R2, and isrotated by seventh and eighth external electric field +E4, −E4,respectively, out of second rest-position direction R2 into a third orfourth deflection direction C and D, respectively, which is rotated withrespect to second first rest-position direction R2 by a third and,respectively, fourth angle +β, −β that is equal and opposite thereto,about the direction of electric field +E2 and −E2, respectively. Themagnitude of third and fourth angle +β and −β, respectively, increasesin response to increasing light intensity, up to a specific maximumvalue βmax, not, however, beyond this, so that third and fourthdeflection directions C and D reach first and second limiting deflectiondirections Cmax and Dmax, respectively, in the case that the magnitudeof third or fourth angle +β and −β, respectively, reaches maximum valueβmax.

FIGS. 5A and 5B show schematic, perspective representations of thedevice of FIG. 5, the plate capacitors and their electric wiring beingomitted for the sake of clarity. Likewise for the sake of clarity, thesituations are shown at the same time in FIGS. 5A and 5B, where electricfields +E1, +E4 are applied, and where fields −E1, −E4 having reversedpolarity are applied to first phase-retardation plate 1 and to lightmodulator 4, although these situations do not occur simultaneously, butrather alternating with the frequency of AC voltage Uo (FIG. 5).

In FIG. 5A, light beam 10 a of high intensity is incident on polarizerP1. This is oriented in such a way that it only allows one lightcomponent having a first input polarization direction EPR1 to pass. Thislight component propagates through first λ/2 phase-retardation plate 1,depending on the polarity of AC voltage U1 (FIG. 1), there being eitherno change in the polarization direction or such a change by an angle+4α, as had already been discussed with reference to FIGS. 2A and 2B.

The light subsequently propagates through second light modulator 4,which, in contrast, to first light modulator 2 of FIGS. 1, 2A, 2B, isnot designed as a λ/2 phase-retardation plate, but rather as a λ/4phase-retardation plate. After being reflected at mirror 5, which isassociated with a phase jump of 180°, the light propagates throughsecond light modulator 4 again, in the reverse direction. Overall,therefore, second light modulator 4 likewise acts on the polarizationdirection of the light beam as a λ/2 phase-retardation plate. Due to thehigh intensity of light beam 10 a, depending on the polarity of ACvoltage U4 (FIG. 4), the fourth indicatrix runs in first or secondlimiting deflection direction Cmax or Dmax, so that, a passage of thelight beam twice through second light modulator 2 is in no way followedby a change in the polarization direction of the high-intensity lightbeam.

The light subsequently traverses first phase-retardation plate 1 in thereverse direction. Thus, at the same time, first phase-retardation plate1 fulfills the function of second phase-retardation plate 2 of FIGS. 1,2A, 2B. In this connection, analogously to FIG. 2A or 2B, depending onthe polarity of AC voltage U1, there is either no change in thepolarization direction or such a change by an angle −4α back into theEPR1 direction, so that, in accordance with the present invention, inboth cases, high-intensity light is filtered out at the analyzer 2.

In FIG. 5B, low-intensity light beam 10 b is incident. Since secondlight modulator 4 likewise acts on the whole, therefore, as a λ/2phase-retardation plate on the polarization direction of the light beam,in accordance with the present invention, completely analogously to thesituation of FIG. 3, it is achieved that the system (or opticalassemblage) of FIG. 5 always allows a very weak light beam 10 b to passthrough independently of the momentary polarity of AC voltage Uo, thetransmission decreasing in response to an increasing angle β, i.e., inresponse to an increase in the light intensity and, at β=45°, reachingthe value zero.

FIG. 6 shows a schematic representation of another specific embodimentof a device of the present invention, the electrical wiring beingomitted. The only difference between this specific embodiment and thatof FIGS. 4, 5A, 5B is that polarizer P1 and analyzer P2 of FIGS. 4, 5A,5B are now jointly formed by a first polarizing beam splitter ST1.Conditions (a) and (b) are also fulfilled in accordance with the presentinvention. A low-intensity light beam comes from the left intopolarizing beam splitter ST1, and one part is transmitted (or allowed topass through) straight ahead, and one part (not shown) is reflecteddownwards. The part that is transmitted straight ahead has inputpolarization direction EPR1. The beam reflected at mirror 5 is incidentfrom the right on polarizing beam splitter ST1. Part of it istransmitted (or allowed to pass through) (not shown), part of it isreflected upwardly as an emergent beam 11, for which the beam splitteracts, at the same time, as an analyzer that is crossed in relation tothe EPR1 direction.

Thus, polarizing beam splitter ST1 acts as a polarizer for incidentlight and, at the same time, as an analyzer that is crossed with respectto the polarizer, for light reflected at mirror 5. This specificembodiment has the advantage over those of FIGS. 4, 5A, 5B that theincident direction of the light beam on mirror 5 may be perpendicular.

Analogously to the explanations regarding FIGS. 1-6, it may easily beshown that, by also using the systems (or optical assemblages) of FIGS.1-6 for a second input polarization direction EPR2 that is perpendicularto first input polarization direction EPR1 and that is able to beproduced, for example, by rotating the polarizer by 90°—the effect ofthe present invention, namely of complete blocking of the passage oflight for high light intensity and of the transmission for low lightintensity, is achieved, it also being necessary in this case to rotateanalyzer P2 by 90°—characterized in each of FIGS. 2A, 2B, 3, 5A and 5Bby a vertical dotted double arrow—so that the condition for an analyzerP2 that is crossed with respect to polarizer P1 is retained.

In addition, one may demonstrate that the effect of the devices of FIGS.1-6 in accordance with the present invention is also achieved when thevalue for n in condition (a) is not zero, but rather 1, 2, 3, . . . ,thus any natural number at all.

In addition, one may demonstrate that the effect of the devices of FIGS.1-6 in accordance with the present invention is also achieved when, incondition (b), it is not the case that second deflection direction B andsecond limiting deflection direction Dmax either both run in parallel toone another or perpendicularly to input polarization direction EPR1, ashad been arbitrarily assumed with reference to FIGS. 1-6, but ratherwhen the alternative is realized whereby first deflection direction Aand first limiting deflection direction Cmax both run in parallel to oneanother or perpendicularly to input polarization direction EPR1.

Utilizing tolerance T=±5° in condition (a), it is possible to achieve,when necessary, that the blocking for case β=45° is not complete, butrather a specific fraction of light beam 10 a is transmitted.

It is not only possible to use the device of the present invention forpreventing integral light intensity under conditions of intenseirradiation. The present invention may be employed just as well tofilter out or attenuate light which comes from an intensely directedlight source and is zonally incident on the device, while a darkerbackground is attenuated to a lesser degree or not at all. Thus, thepresent invention may be used as a filter to protect the eye from alaser beam, while the background remains viewable through the filter.

The devices of FIGS. 1-6 may be equipped with a lens which images afield-of-view onto first or second light modulator 2, 4, and with aneyepiece which is positioned in such a way as to enable the image of thefield-of-view to be observed using the eyepiece. Together with thedevices of FIGS. 1-6, the lens and eyepiece then act as a telescope,which, in accordance with the present invention, darkens intenselyluminous zones of the field-of-view, while the remaining zones are notdarkened at all or are only darkened little. If such a telescope isdirected at an intense light source against a darker background, forexample at the sun surrounded by blue sky, then the very intense lightof the sun is substantially darkened or completely filtered out, while,at the same time, the background is able to be viewed as in a nearlyfull transmission. Of course, it may be necessary to introduce measuresto prevent a too intense local or integral heating of the lightmodulator, in particular.

FIG. 7 shows a cross-sectional representation of one specific embodimentof the present invention, where the need is advantageously eliminatedfor filtering out one polarization direction in response to theincidence of light on the device. The device of FIG. 7 includes firstλ/2 phase-retardation plate 1, second optically addressable, spatiallyresolving light modulator 4 designed as a λ/4 phase-retardation plate,and mirror 5 of FIG. 4, as well as, additionally, a lens 22, an eyepiece23, two light-deflecting surfaces 8, 9, which may be mirrors, forexample, or mirrored prisms, and a second polarizing beam splitter ST2,which functions as a polarizer for incident light 10 and, at the sametime, as an analyzer that is crossed with respect to the polarizer, forlight reflected at mirror 5.

In the same way as in FIG. 4, first λ/2 phase-retardation plate 1 orlight modulator 4 is situated inside of plate capacitors C1 and C4,respectively, in electric fields, whose polarity is reversed, insynchronous, cyclical fashion from +E1 to −E1 and from +E4 to −E4,respectively, and vice versa, the electric wiring not being shown.Polarizing beam splitter ST2, first phase-retardation plate 1, and thelight modulator are positioned in such a way in accordance with thepresent invention that conditions (a) and (b) are met.

Lens 22 images a field-of-view onto second light modulator 4. The imageof the field-of-view is able to be observed using eyepiece 23. From thefield-of-view, unpolarized light 10, for example, is incident on thedevice. In second polarizing beam splitter ST2, this light is dividedinto two beam components T1, T2, which are linearly polarizedperpendicularly to one another, first beam component T1 being deflectedand second beam component T2 not being deflected. For example, firstbeam component T1 may be polarized perpendicularly to the plane of thepaper of FIG. 6, and second beam component T2 may be polarized inparallel to the same. First and second beam components T1, T2 are eachdeflected by one of light-deflecting surfaces 8, 9 in such a way thatthey pass separately from one another through first λ/2phase-retardation plate 1 and second light modulator 4 and strike mirror5, and, from there, pass again, in reverse order, through second lightmodulator 4 and first λ/2 phase-retardation plate 1 and, by way oflight-deflecting surfaces 8, 9, back into polarizing beam splitter ST2.

There, first beam component T1 is divided into a third beam componentT11, which is linearly polarized in parallel to first beam component T1and is deflected in polarizing beam splitter ST2 in the direction oflens 23, and into a fourth beam component T12, which is linearlypolarized perpendicularly to first beam component T1 and is notdeflected in polarizing beam splitter ST2.

After being reflected at mirror 5 and entering once again into beamsplitter ST2, second beam component T2 is divided into a fifth beamcomponent T21, which is linearly polarized perpendicularly to secondbeam component T2 and is deflected in polarizing beam splitter ST2 inthe direction of fourth beam component T12, and into a sixth beamcomponent T22 which is linearly polarized in parallel to second beamcomponent T2 and is not deflected in polarizing beam splitter ST.

Second light modulator 4 is situated in the focal plane of lens 22.Since incident light 10 is divided into two independent components T1,T2, which are polarized perpendicularly to one another, two spaced-apartimages of the field-of-view are formed on light modulator 4. Eyepiece 23is positioned in such a way that fourth and fifth beam components T12,T21 arrive at eyepiece 23, so that both images of the field-of-view maybe viewed through eyepiece 23, the polarization information of thefield-of-view being completely contained in beam components T12, T21.Polarizing beam splitter ST2 is preferably formed and positioned in sucha way that the path of fourth beam component T12 coincides with the pathof fifth beam component T21, so that, for an observer, both images ofthe field-of-view are coincident.

When the intensity of incident light 10 is very substantial, inaccordance with the present invention, the intensity of the two beamcomponents T12, T21 approaches zero. Accordingly, the device of FIG. 7functions as a telescope, which advantageously acts at the same timeboth on the component of incident light 10 that is polarizedperpendicularly to the plane of the paper, as well as on the componentof incident light 10 that is polarized in parallel to the plane of thepaper, in a way that enables only intensely luminous zones of thefield-of-view to be darkened, while the remaining zones are not darkenedor are darkened only little. It is beneficial that, even when veryintense light sources are viewed, beam splitter ST2 is subjected to onlylittle thermal loading since it also divides the light reflected atmirror 5 merely into beam components T11 and T12, respectively, T21 andT22; beam components T11 and T22, which, given a high intensity ofincident light 10, in turn, have a high intensity, exiting the device tothe outside, through the lens and, thus, substantially contributing tothe dissipation of energy out of the device.

This effect according to the present invention is illustrated in, e.g.,FIG. 7 by two objects S1, F1 of the field-of-view, S1 emitting light ofhigh intensity and F1 light of low intensity. S1 can be the sun, forexample, and F1 an airplane. The observer, for example the pilot ofanother airplane, who would like to observe airplane F1 against thelight of the sun, sees image F2 of airplane F1, darkened only slightlyin eyepiece 23, while he sees image S2 of sun S1 advantageously darkenedquite substantially, so that the device of the present invention makesit easier, or even possible in the first place, for him to observeairplane F1.

A variation of the device of FIG. 7 is shown in FIG. 8, where, insteadof second polarizing beam splitter ST2 of FIG. 7, a third polarizingbeam splitter ST3 is used, which is formed in such a way that two of itsbounding surfaces act as light-deflecting surfaces 8′, 9′, which deflectfirst and second beam components T1, T2 in the direction of mirror 5,and, once reflected at mirror 5, back into beam splitter ST3. Lightdeflecting surfaces 8′, 9′ are preferably externally reflecting.

It is merely for the sake of clarity that beam components T11, T22 andT21, T22 are sketched in such a way in FIGS. 7 and 8 so as to be offsetfrom one another.

FIG. 9A shows another specific embodiment of a device according to thepresent invention, viewed transversely to incoming light beam 10 b andtransversely to emergent light beam 11, the electrical wiring beingomitted. FIG. 9B shows the device of FIG. 9A, viewed in the direction ofincident light beam 10 b; and FIG. 9C shows this device viewed in theopposite direction of emergent light beam 11.

Polarizing beam splitter ST1 divides incident light 10 b into twomutually perpendicular, linearly polarized beam components. They emergeto the right and, respectively, in a downwards direction from beamsplitter ST1 in FIG. 9A, and strike light-deflecting surfaces 8″, 9″,respectively, which are each tilted by 45° with respect to the directionof propagation of the beam component in question, in order to deflectthe beam components in the direction of phase-retardation plate 1. Themutual configuration of the phase-retardation plate, light modulator 4,and of the mirror, and the configuration of plate capacitors C1, C4 andtheir electrical wiring correspond to those of FIG. 7. The onlydistinction is that light-deflecting surfaces 8″, 9″ and polarizing beamsplitter ST1 of FIGS. 9A, B, C are positioned differently thanlight-deflecting surfaces 8, 9 and polarizing beam splitter ST2 of FIG.7.

The present invention can have industrial applicability, for example, inthe area of industrial safety, welding processes, aviation security,pyrotechnics and medical technology.

1. An intensity-dependent attenuation device for light, having a polarizer, which allows linearly polarized light to propagate through in one input polarization direction, and having an analyzer which is crossed relatively thereto by one of a substantial number of degrees and 90 degrees, comprising: a λ/2 phase-retardation plate, an optically addressable, spatially resolving light modulator designed as a λ/4 phase-retardation plate, and a mirror, incident light propagating, in turn, through the polarizer, the λ/2 phase-retardation plate, and the optically addressable, spatially resolving light modulator, and, from there, impinging on the mirror, and, subsequently, in the reverse direction, again propagating through the optically addressable, spatially resolving light modulator and the λ/2 phase-retardation plate and, from there, impinging on the analyzer, the λ/2 phase-retardation plate having a first indicatrix which without an external electric field, runs in a first rest-position direction; in response to the application of a first external electric field, runs in a first deflection direction, which is rotated with respect to the first rest-position direction by a first angle that is independent of the light intensity, about the direction of the first external electric field; and in response to the application of a second external electric field, equal and opposite to the first external electric field, runs in a second deflection direction, which is rotated with respect to the first rest-position direction by a second angle, which is equal and opposite to the first angle; the optically addressable, spatially resolving light modulator having a second indicatrix, which without an external electric field, runs in a second rest-position direction; in response to the application of a third external electric field, runs in a third deflection direction, which is rotated with respect to the second rest-position direction by a third angle, about the direction of the third external electric field; in response to the application of a fourth external electric field, equal and opposite to the third external electric field, runs in a fourth deflection direction, which is rotated with respect to the second rest-position direction by a fourth angle, equal and opposite to the third angle; the magnitude of the third and of the fourth angle increasing in response to increasing light intensity, up to a specific maximum value, not however beyond this, and the third and fourth deflection directions reaching a first and second limiting deflection direction, respectively, in the case that the magnitude of the third or fourth angle reaches the maximum value; c) the phase-retardation plate and the optically addressable, spatially resolving light modulator being so oriented to one another and with respect to the polarizer that the first deflection direction and the first limiting deflection direction run in parallel to one another and in parallel or perpendicularly to the input polarization direction, or the second deflection direction and the second limiting deflection direction run in parallel to one another and in parallel or perpendicularly to the input polarization direction; the first or second external electric field being applied to the phase-retardation plate, and the third or fourth external electric field being applied to the optically addressable, spatially resolving light modulator; and the first and second, and the third and fourth external electric fields, respectively, being selected in such a way that the first angle and the maximum value fulfill the condition 2α−βmax=n·45°+T, n being equal to 0,1,2,3, . . . , α being the first angle, βmax being the maximum value, and T being a tolerance of ±5°.
 2. The device as recited in claim 1, wherein the λ/2 phase-retardation plate, the optically addressable, spatially resolving light modulator, and the mirror are positioned in parallel with one another.
 3. The device as recited in claim 1, wherein the first and the second external electric fields, respectively, are generated by a first plate capacitor, between whose plates the phase-retardation plate is situated; and the third and fourth external electric fields, respectively, are generated by a second plate capacitor, between whose plates the optically addressable, spatially resolving light modulator is situated.
 4. The device as recited in claim 3, wherein an AC voltage having a substantially square-wave time characteristic is applied to each of the plate capacitors, zero crossings of the AC voltages occurring simultaneously.
 5. The device as recited in claim 3, wherein the plates of the plate capacitors are constituted of electrically conductive layers which are at least partially reflecting to light and which are directly placed on the light-transit surfaces of the phase-retardation plate or of the optically addressable, spatially resolving light modulator, or are placed at a distance, in parallel to the same, using a transparent substrate material.
 6. The device as recited in claim 5, wherein the layers are made of a thin layer of metal.
 7. The device as recited in claim 5, wherein the layers are made of indium tin oxide.
 8. The device as recited in claim 4, wherein the AC voltages originate from a shared voltage source.
 9. The device as recited in claim 1, wherein the polarizer and the analyzer are jointly formed by a polarizing beam splitter which functions as a polarizer for incident light and, at the same time, as an analyzer for light reflected at the mirror.
 10. The device as recited in claim 9, wherein the incident light is divided by the polarizing beam splitter into a linearly polarized first beam component, which is deflected in the polarizing beam splitter, and into a second beam component, which is linearly polarized perpendicularly to the first beam component and is not deflected in the polarizing beam splitter, the first and the second beam components passing separately from one another through the λ/2 phase-retardation plate and the optically addressable, spatially resolving light modulator and striking the mirror, and, from there, passing again, in reverse order, again through the optically addressable, spatially resolving light modulator and the λ/2 phase-retardation plate and arriving again in the polarizing beam splitter, where the first beam component is divided into a third beam component, which is linearly polarized in parallel to the first beam component and is deflected in the polarizing beam splitter, and into a fourth beam component, which is linearly polarized perpendicularly to the first beam component and is not deflected in the polarizing beam splitter, and the second beam component is divided into a fifth beam component, which is linearly polarized perpendicularly to the second beam component and is deflected in the polarizing beam splitter, and into a sixth beam component which is linearly polarized in parallel to the second beam component and is not deflected in the polarizing beam splitter.
 11. The device as recited in claim 10, wherein the polarizing beam splitter is formed and positioned in such a way that the path of the fourth beam component coincides with the path of the fifth beam component.
 12. The device as recited in claim 10, wherein positioned between the polarizing beam splitter and the λ/2 phase-retardation plate is at least one light-deflecting surface, which deflects the first and the second beam component, respectively, in such a way as to enable the first and the second beam component to run in parallel with one another.
 13. The device as recited in claim 10, wherein the polarizing beam splitter has at least one light-deflecting surface which deflects the first and the second beam components, respectively, in such a way that the first and the second beam components run in parallel to one another on the path from the light-deflecting surface to the mirror.
 14. The device as recited in claim 1, wherein a collimator is placed upstream from the mirror.
 15. A method for attenuating light as a function of intensity, which passes through a polarizer, which allows linearly polarized light to propagate through in one input polarization direction and strike an analyzer that is crossed thereto by a substantial number of degrees or by 90 degrees, characterized by a λ/2 phase-retardation plate, an optically addressable, spatially resolving light modulator designed as a λ/4 phase-retardation plate, and a mirror, incident light propagating, in turn, through the polarizer, the λ/2 phase-retardation plate, and the optically addressable, spatially resolving light modulator, and, from there, impinging on the mirror, and, subsequently, in the reverse direction, again propagating through the optically addressable, spatially resolving light modulator and the λ/2 phase-retardation plate and, from there, impinging on the analyzer, the λ/2 phase-retardation plate having a first indicatrix which without an external electric field, runs in a first rest-position direction; in response to the application of a first external electric field, runs in a first deflection direction, which is rotated with respect to the first rest-position direction by a first angle that is independent of the light intensity, about the direction of the first external electric field; and in response to the application of a second external electric field, equal and opposite to the first external electric field, runs in a second deflection direction, which is rotated with respect to the first rest-position direction by a second angle, equal and opposite to the first angle; the optically addressable, spatially resolving light modulator having a second indicatrix, which without an external electric field, runs in a second rest-position direction; in response to the application of a third external electric field, runs in a third deflection direction, which is rotated with respect to the second rest-position direction by a third angle, about the direction of the optically addressable, spatially resolving external electric field; in response to the application of a fourth external electric field, equal and opposite to the third external electric field, runs in a fourth deflection direction, which is rotated with respect to the second rest-position direction by a fourth angle, equal and opposite to the third angle; the magnitude of the third and of the fourth angle increasing in response to increasing light intensity, up to a specific maximum value, not however beyond this, and the third and fourth deflection directions reaching a first and second limiting deflection direction, respectively, in the case that the magnitude of the third or fourth angle reaches the maximum value; the phase-retardation plate and the optically addressable, spatially resolving light modulator being so oriented to one another and with respect to the polarizer that the first deflection direction and the first limiting deflection direction run in parallel to one another and in parallel or perpendicularly to the input polarization direction, or the second deflection direction and the second limiting deflection direction run in parallel to one another and in parallel or perpendicularly to the input polarization direction; the first or the second external electric field being applied to the phase-retardation plate, and the third or fourth external electric field being applied to the optically addressable, spatially resolving light modulator; and the first and the second, and the third and the fourth external electric fields, respectively, being selected in such a way that the first angle and the maximum value fulfill the condition 2α−βmax=n·45°+T, n being equal to 0,1,2,3, . . . , α being the first angle, βmax being the maximum value, and T being a tolerance of ±5°.
 16. The method as recited in claim 15, wherein the first and the second external electric fields, respectively, are generated by a first plate capacitor, between whose plates the phase-retardation plate is situated; and the third and fourth external electric fields, respectively, are generated by a second plate capacitor, between whose plates the optically addressable, spatially resolving light modulator is situated.
 17. The method as recited in claim 16, wherein an AC voltage having a substantially square-wave time characteristic is applied to each of the plate capacitors, zero crossings of the AC voltages occurring simultaneously.
 18. The method as recited in claim 17, wherein the AC voltages originate from a shared voltage source.
 19. The method as recited in claim 15, wherein the polarizer and the analyzer are jointly formed by a polarizing beam splitter which functions as a polarizer for incident light and, at the same time, as an analyzer for the light reflected at the mirror.
 20. The method as recited in claim 19, wherein the incident light is divided by the polarizing beam splitter into a linearly polarized first beam component, which is deflected in the polarizing beam splitter, and into a second beam component, which is linearly polarized perpendicularly to the first beam component and is not deflected in the polarizing beam splitter, the first and the second beam components passing separately from one another through the λ/2 phase-retardation plate and the optically addressable, spatially resolving light modulator and striking the mirror, and, from there, passing again, in reverse order, again through the optically addressable, spatially resolving light modulator and the λ/2 phase-retardation plate and arriving again in the polarizing beam splitter, where the first beam component is divided into a third beam component, which is linearly polarized in parallel to the first beam component and is deflected in the polarizing beam splitter, and into a fourth beam component, which is linearly polarized perpendicularly to the first beam component and is not deflected in the polarizing beam splitter, and the second beam component is divided into a fifth beam component, which is linearly polarized perpendicularly to the second beam component and is deflected in the polarizing beam splitter, and into a sixth beam component which is linearly polarized in parallel to the second beam component and is not deflected in the polarizing beam splitter.
 21. The method as recited in claim 10, wherein the path of the fourth beam component coincides with the path of the fifth beam component.
 22. The method as recited in claim 15, wherein a lens images a field-of-view onto the light modulator. 